The present invention pertains to hydraulically actuated transmissions having various change speed gear ratios, the gears of which are selectively actuated by hydraulically operted friction clutches. These prior art transmissions have had some shortcomings such as the inability to insure that smooth shifting between speed ranges occurs and particularly that only one speed range at a time is selected, and before one speed range is engaged all others are released. Other shortcomings of these prior transmissions have occured when electric power or logic failure occurs and the transmission is left in a particular speed range in one direction or another; a malfunction of the parts may occur when it is attempted to again start the transmission with a particular speed range in the engaged position.
One example of a prior art arrangement for hydraulic gear shifting and driving system is shown in the U.S. Pat. No. 2,360,695 issued Oct. 17, 1944 to Linden et al. That device used grooved cylindrical rotating elements with indexing means for moving the major gear change element through a portion of a revolution while the minor gears change valve element is completely revolved. This device used a geneva gear element with nontangential notches relative to the path of the driving arm pin which produces shock during transmission of force and limited the device to very slow shifts to prevent premature failure of the parts. It also had a complex detent roller and pivoted arm arrangement for locking against rotation.
Another example of the prior art is the U.S. Pat. No. 2,498,189 issued Feb. 21, 1950 to Wattson for automatic control and distribution of fluids. That device used a gear motor with a large speed reduction driving a pinion having an incomplete complement of circularly spaced teeth and a secondary gear and a distribution gear, all of which resulted in very slow responding shifts and relies on friction to prevent the bell from over-shooting a fluid outlet or drifting from its position at the fluid outlet, since it lacked the positive means for indexing and holding the bell in position.